Apparatus for expressing writing fluid from a plotting pen

ABSTRACT

In a plotting device in which a plotting pen and a recording medium move relative to one another while the pen produces a line trace on the medium, a fluid pump is connected to the plotting pen and forcibly expresses writing fluid from the pen at a rate proportional to the rate of relative movement between the pen and the recording medium. In one such plotting device, a recording medium such as a strip of plotting paper, is transported between two supply rolls by means of a motor-driven plotting drum. The plotting pen is transported laterally of the strip of plotting paper and over the plotting drum by means of a motor-driven carriage. The motors of the plotting drum and the carriage receive plotting commands from a digital computer and a velocity signal circuit in the plotting device derives a velocity signal proportional to the relative movement generated by the motors. The velocity signal is utilized to control the speed of the fluid pump which is specially constructed to have a linear speed versus flow rate characteristic.

1 Dec. 25, 1973 I APPARATUS FOR EXPRESSING WRITING FLUID FROM A PLOTTINGPEN FOREIGN PATENTS OR APPLICATIONS 637,462 5 1950 Great Britain 346/140i coMPuTERj }/92 90 Y x RATE RATE 1! i 94 VELOCITY CIRCUIT l 1 PUMP PUMPDRIVER MOTOR 7o io x DRIVER |-1 x MOTOR]- J I CLEARING |-1 PrimaryExaminer.loseph W. I-Iartary Att0rney-J0hn C. Linderman [57] ABSTRACT Ina plotting device in which a plotting pen and a recording medium moverelative to one another while the pen produces a line trace on themedium, a fluid pump is connected to the plotting pen and forciblyexpresses writing fluid from the pen at a rate proportional to the rateof relative movement between the pen and the recording medium. In onesuch plotting device, a recording medium such as a strip of plottingpaper, is transported between two supply rolls by means of amotor-driven plotting drum. The plotting pen is transported laterally ofthe strip of plotting paper and over the plotting drum by means of amotor-driven carriage. The motors of the plotting drum and the carriagereceive plotting commands from a digital computer and a velocity signalcircuit in the plotting device derives a velocity signal proportional tothe relative movement generated by the motors. The velocity signal isutilized to control the speed of the fluid pump which is speciallyconstructed to have a linear speed versus flow rate characteristic.

10 Claims, 7 Drawing Figures W l l i l PATENTEU 3.781.907

suI1of3 f I COMPUTER v v 3 l Y x RATE RATE Y 94 I VELOCITY/ CIRCUIT T iGE 1 1 PUMP D PUMP 1 DRIVER MOTOR GEARING i I I I Y DRIVER Y MOTOR FIG.2

APPARATUS FOR EXPRESSING WRITING FLUID FROM A PLOTTING PEN BACKGROUND OFTHE INVENTION The present invention relates to recording or motordrivenplotting devices in which a plotting pen and recording medium are movedrelative to one another to produce line traces defining graphicalinformation. More particularly, the invention relates to apparatus forexpressing a writing fluid from a plotting pen so that the quality ofthe traces and the graphic information does not deteriorate at higherplotting speeds.

As the state of the art in the field of plotting devices progresses andthe maximum speeds of the plotting devices increase, the transfer of thewriting fluid from the plotting pen onto the recording medium becomes aproblem. As the plotting speed increases beyond the capacity of astandard wetting pen through which the writing fluid is drawn bycapillary action, the character of the lines produced by the pendeteriorates. The intensity and width of the lines decrease as thequantity of writing fluid deposited per unit length of line decreases.To limit the deterioration in quality, pressurized ball point pens havebeen employed; however, due to limitations of the ball point, such asits tendency to skip, a high quality printout having high contrast andclarity cannot be achieved at high plotting speeds, for example, 700inches per minute and higher. Also, uniform line characteristics atvarious plotting speeds are desirable. Even though the traces may beclear, variations in the line widths due to different plotting speedsmay be unacceptable in, for example, making master transparencies forprinted circuit components.

It is, accordingly, a general object of the present invention todisclose apparatus for expressing a writing fluid from a plotting pen toproduce uniform, traces at different plotting speeds and high qualitytraces at higher plotting speeds.

SUMMARY OF THE INVENTION The present invention resides in apparatus forexpressing writing fluid from a plotting pen onto a recording mediumwhile the pen and recording medium are moved relative to one another ina plotting device. In such plotting devices, command signals generatedfrom on line" data or data derived from memory devices such as magneticor punched paper tape are supplied to the plotting device to produce aprescribed relative movement between the plotting pen and the recordingmedium.

The apparatus of the present invention comprises velocity signalgenerating means'which is connected with the plotting device andproduces a.velocity signal representative of the rate of movement of thepen and the recording medium relative to one another in the plottingdevice. The signal generating means may include a rate sensor, such as apulse rate tachometer, which receives the command signals supplied tothe plotting device. The apparatus also includes fluid pumping meansconnected to the plotting pen and the velocity signal generating meansand which responds to the velocity signal to force writing fluid fromthe plotting pen at a flow rate corresponding with the rate of movementof the pen and recording medium relative to one another in the plottingdevice. I

The pumping means comprises a fluid pump having a discharge ductconnected to the plotting pen to deliver writing fluid to the pen, and apump motor driven at a rate proportional to the velocity signal producedby the generating means. The pump is connected to the motor through agear reduction unit and operates at a low speed consistent with thesmall flow rates required of a plotting pen. The pump employsreciprocating pistons which are driven by means of cams that provide alinear speed versus a flow rate characteristic. By driving the pumphaving such linear flow rate characteristics at a speed proportional toa desired flow rate, more particularly, a flow rate directlyproportional to the speed of relative movement between the pen and therecording medium, line traces on the recording medium retain the samehigh quality at all plotting speeds. The plotting pen is a wetting-typepen which has a capillary tube at the writing tip through which thewriting fluid is forced during the plotting operation. The pen isprovided with an electrically oscillated filament to clean the capillarytube as needed.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of aplotting device embodying the apparatus for expressing writing fluidfrom the plotting pen in accordance with the present invention.

FIG. 2 is a schematic diagram showing control components within theplotting device shown in FIG. 1.

FIG. 3 is a cross-sectional view of a fluid pump utilized in theplotting device of FIG. 1.

FIG. 4 is an enlarged sectional view of the fluid pump taken along theline 4-4 of FIG. 3.

FIG. 5 is an end elevational view of the fluid pump shown in FIG. 3.

FIG. 6 is a sectional view of the fluid switching valve taken along theline 6-6 in FIG. 3.

FIG. 7 is a cross-sectional view of the plotting pen utilized in theplotting device in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows in a perspectiveview a plotting device, generally designated 10, in which a plotting penl2 and a recording medium 14 are moved relative to one another while awriting fluid is expressed from the pen onto the medium I4 at a rateproportional to the relative movement or velocity between the pen 12 andthe medium 14. The plotting device 10 receives plotting commands from adigital computer 20 as the computer reads information from a digitalmemory tape 22. The particular operation performed by the computer inprocessing the information from the tape 22 forms no part of the presentinvention and it is sufficient for understanding the present inventionto state that after utilizing the information from the tape, thecomputer transmits command signals which control the operation of theplotting device 10 through a command signal cable 24 and cause specifiedgraphic information to be recorded on the medium. The command signalsmay be in either digital or analog form, but for the purposes ofdescription in the present specification, it will be assumed thatdigital signals are employed.

The recording medium 14, which is shown as a strip of plotting paper, istrained over a plotting drum 30 which engages the strip with sprocketsor other means and translates the strip longitudinally of itself backand forth between two supply rolls 32 and 34 located parallel to thedrum and at opposite sides of the drum. The

drum 30 is mounted for rotation in the housing 38 of the plotting deviceby means bearings (not shown) and is rotated by a'variable speed drivemotor 40 connected directly to the mounting shaft for the drum. Thesupply rolls 32 and 34 are also rotatably mounted in the housing 38 andare driven in opposition to one another by torquing motors (not shown)so that the strip of the recording medium 14 trained over the plottingdrum remains taut as the medium is paid on and off the two supply rolls.

The plotting pen 12 is mounted to a plotting head 50 so that the writingtip of the pen 12 can be positioned in writing contact with therecording medium 14 in substantially perpendicular relationship to themedium resting on the plotting surface of the drum 30. For this purposethe pen 12 is mounted on a cantilevered arm 52 which is pivotallymounted in the head 50 so that the pen 12 can be lowered into writingcontact with the recording medium 14 or lifted away from the medium whenno trace is desired. The plotting head 50 is supported on a pair ofparallel rails 54 and 56 fixedly mounted to the housing 38 and extendingparallel to the axis of rotation of the plotting drum 30 so that theplotting head 50 and the pen 12 can be moved transversely of therecording medium 14. A variable speed drive motor 60 is connected indriving relationship to the head 50 by means of a toothed drive belt 62so that the motor 60 causes the head 50 and pen 12 to move transverselyof the recording medium 14 while the drive motor causes the plottingdrum 30 to move the recording medium 14 back and forth in thelongitudinal direction. It will be understood that the drive motors 40and 60 cause the recording medium 14 and the plotting head with the pen12 to be moved relative to one another along mutually perpendiculardirections, the longitudinal direction in which the recording medium ismoved by the plotting drum 30 and the motor 40 being normally associatedwith an X command axis and the lateral direction along which theplotting head 50 and pen 12 are moved relative to the medium beingassociated with a Y command axis. The pivotal motions of thecantilevered arm 52 which raise and lower the pen 12 over the recordingmedium are designated Z axis commands since the vertical motions of thepen 12 are generally perpendicular to the longitudinal motions of therecording medium 14 and the lateral motions of the pen 12.

Turning briefly to FIG. 2, the control signal channels from the digitalcomputer 20 to the components associated with the X, Y, and Z axes ofthe plotting device 10 are shown in a schematic diagram. Motion commandsin the form of digital motor pulses and sign information are transmittedrespectively to X and Y drivers 70 and 72. The drivers 70 and 72 may beformed by digital-to-analog servocontrols such as disclosed in theco-pending application Ser. No. 169,263, filed Aug. 5, 1971, having thesame assignee as the present application. The drivers 70 and 72 applyelectrical drive signals to the variable speed drive motors 40 and andthe motors accordingly produce the relative movement between the pen 12and the recording medium 14. The Z axis commands are transmitted fromthe digital computer 20 to a Z axis driver 74 and actuate a solenoid 76to cause the pen 12 to be pivotally lowered against the plotting drum 30and in contact with the recording medium translated on the plottingdrum. The Z axis driver 74 may be a simple transistor amplifier and thepen 12 may be biased upwardly out of contact with the recording medium14 by means of a biasing spring (not shown). it will be understood thatby appropriate commands to the drivers 70, 72, 74, the plotting pen 12and recording medium 14 are moved relative to one another so that thewriting tip of the pen may be stroked over the recording medium in acontrolled manner and produce any type of graphic or trace includinggeometric figures, letters, numbers and curves.

As shown in FIGS. 1 and 2, the pen 12 is supplied with writing fluid bymeans of a fluid pump 80 which is connected to the pen through aflexible tube 82. The fluid pump 80 is operated by a pump motor 84through a gear reduction unit 86. The pump 80, the pump motor 84 and thegear unit 86 are all mounted to the plotting head 50 of the plottingdevice 10. The common mounting of the pen, pump and motor on the head 50permits the interconnecting flexible tube 82 to be relatively short andeliminates compliance in the conduits connecting the pump and the penwhich compliance might produce transients affecting the flowrates of thewriting fluid and the quality of the line traces on the recording medium14. The reservoir for the writing fluid may be embodied in the housingof the pump 80 or mounted separately to the plotting device 10.

The control signals which regulate the flow of writing fluid from thepump 80 are derived from the command signals applied to the drivers and72 by means of X and Y pulse rate tachometers 90 and 92 respectively.

The tachometer 90 detects the motor pulse rate of the a command signalsapplied to the X driver 70 and the tachometer 92 detects the motor pulserate of the command signals applied to the Y driver 72. The pulse ratesignals from the tachometers 90 and 92 are applied to a velocity signalgenerating circuit 94 which continuously computes the total relativevelocity of the plotting pen 12 and recording medium 14. The generatingcircuit 94 produces a signal proportional to the total velocity andapplies that signal to a pump driver 96, which may be a standardvariable gain, closed loop amplifier, that operates the pump motor 84.The driver 96 also receives a feedback signal proportional to the speedof the pump motor 84 from a motor tachometer 98. The pump motor 84 is,therefore, operated in a closed loop servosystem which maintains theoutput speed of the motor 84, and also the gearing 86, proportional tothe relative velocity of the recording pen l2 and recording medium 14.The pump is designed to have linear flow versus speed characteristics sothat the writing fluid flows from the pump 80 through the plotting pen12 and is expressed from the writing tip onto the recording medium 14 ata rate which is proportional to the relative velocity of the pen in themedium. The gain of the pump driver 96 is made adjustable so that theproportional flow rate characteristic of the fluid pumping system can bevaried for different pen tip sizes, writing fluid viscosities, paperabsorption characteristics and line characters, that is heavy or light.The driver may be designed so that the adjustment of its gain is eithermanually effected or is controlled directly from the digital computer20.

It is important that the fluid pump 80 have the linear flow versus speedcharacteristic and be able to produce a continuous flow of writing fluidat the controlled rate. If the linear flow characteristics are notsubstantially met, the character and quality of the line traces on therecording medium may vary with undesirable results at different plottingspeeds. If the pump is not capable of producing a smooth and continuousflow of writing fluid, the trace on the recording paper may be broken atstations where the flow rate is momentarily interrupted.

One embodiment of the pump 80 which has the necessary characteristics isshown in FIGS. 3-6. The pump is a piston pump and includes a housing 110in which fluid displacement chambers are formed between the ends of twocylinders 112 and 114 and two reciprocating pistons 116 and 118 mountedrespectively in the cylinders 112 and 114. The pistons 116 and 118 aredriven respectively by cams 120 and 122 which are fixedly mounted to therotatable drive shaft 124 by means of clamps 126 and 128 respectively.The drive shaft 124 is rotatably mounted in the upper portion of thehousing 110 by means of clamps 130 and 132 and is driven by the gearunit 86 through a gear set 134. Cam followers 136 and 138 pinned to theupper ends of pistons 116 and 118 respectively engage the cams 120 and122. As shown in FIG. 4, the cam follower 136 is biased upwardly intocontact with the cam 120 by means of two return springs 140 and 142mounted in parallel relationship with the piston 116. The piston 116 istherefore reciprocatedin the cylinder 112 by the cam 120 and the returnsprings 140 and 142 as the cam follower 136 engages the profile of cam120. The upper end of the piston 116 is sealed by means of an 'O-ringseal 144. The piston 118 is operated by the cam 122 in the same manner.

The profile of the cam 120 shown in FIG. 4 and similarly the profile ofthe cam 122 are designed to cause the pistons 116 and 118 to move in areciprocating manner within the cylinders 112 and 114 and to reduce thevolumes of the displacement chambers in the cylinders by amountsproportional to the rotation of the shaft 124 during overlappingportions of the reciprocation cycles. More specifically, the segment ofthe profile on the cam 120 which provides the proportional displacementof the piston 116 and reduction of the fluid displacement chambervolumes as the drive shaft 124 rotates in the direction of the arrow bsubtends the arch a greater than 180 as shown in FIG. 4. The remainingsegment of the cam profile permits the piston 116 to move upwardly tothe phantom position of the follower 136 indicated in FIG. 4 and ingestwriting fluid for the next pumping stroke. The end portions of theremaining cam profile segment may provide a dwell in the piston motions.The profile of the cam 122 is the same as that on cam 120. The cams 120and 122 are fixedly mounted to the drive shaft 124 at rotationalpositions which are substantially 180 apart so that the proportionalstroking segment of at least one or the other of the cams is engagedwith a cam follower at each rotational position of the drive shaft 124and, therefore, at least one of the reciprocating pistons will alwaysprovide a proportional reduction in the displacement chamber volume anda flow rate proportional to shaft speed.

In order to connect the cylinders 112 and 114 to the plotting pen 12 atthe appropriate times during the reciprocation cycles of the pistons, aswitching valve 150 is connected between the fluid passageways 152 and154 leading respectively to the cylinders 112 and 114 and the dischargeand inlet ducts 156 and 158 of the pump. The switching valve 150 is anelectrically actuated valve comprised of a mounting plate 160 and arotatable plate 162 which contain porting ducts mating with thepassageways 152, 154 and the ducts 156 and 158. The plates 160 and 162have contacting, lapped surfaces which are clamped together to preventleakage of the writing fluid and the plate 162 is rotated relative tothe plate 160 by means of a rotary solenoid 164 and a gear 166. Aportion of the outer periphery of the plate 162 bears gear teeth matingwith the gear 166 as shown most clearly in FIG. 6.

The inlet duct 158 is a flexible duct and connects with the writingfluid reservoir 170 to supply fluid to the displacement chambers of thepump. The reservoir 170 is preferably mounted on the plotting head withthe pump and may take the form of a cartridge or collapsible container.The discharging duct 156 is also flexible and connects directly with theduct 82 leading to the plotting pen as shown in FIG. 1.

As shown in FIG. 6, the rotatable plate 162 of the switching valve 150contains two arcuate cutouts 180 and 182 into which the inlet duct 158and discharge duct 156 are connected respectively. The stationary plate160 mounted to the housing of the pump contains conduits 172 and 174which connect directly through the passageways 152 and 154 respectivelyto the cylinders 112 and 114. When the rotary solenoid 164 rotates thegear 166, the rotatable plate 162 is moved through an arch 0 so that thearcuate cutouts 180 and 182 are moved between the positions shown andthe phantom positions indicated. As a result, the cylinder 112 connectswith the arcuate cutoff 180 at one position of the rotatable plate 162and with the other arcuate cutout 182 at the other position of the plate162. Conversely, the cylinder 114 connects with the arcuate cutout 182at the one position of the plate 162 and with the cutout 180 at theother position of the plate 162. Accordingly, the inlet and dischargeducts 158 and 156 are alternately connected to the cylinder 112 insequence and at the same time are alternately connected to the cylinder114 in the reverse sequence.

It will be readily understood that since the cams and 122 displace thepistons 116 and 118 at proportional rates during a portion of the driveshaft rotation greater than 180 and since the cams are phased on thedrive shaft 124 so that at least one of the cams is displacing one ofthe pistons in proportional fashion at each rotational position of thedrive shaft 124, a flow rate proportional to the shaft rotation can bederived from the discharge conduit 156 at each rotational position ofthe drive shaft provided that the switching valve is actuated atappropriate times during each revolution of the drive shaft. To thisend, a switching cam 190 shown in FIGS. 3 and 5 is fixedly mounted tothe one end of the drive shaft 124 and a microswitch 192 actuated by thecam 190 is secured to the housing 110 of the pump 80. The profile of thecam 190 causes the microswitch 192 to close during the of rotation ofthe drive shaft 124 in which the cam 120 displaces the piston 116 andfluid within cylinder 112, and in this manner actuates the rotary relay164 to cause the switching valve 150 to be positioned so that thedischarge port 156 and the cylinder 112 are in fluid communication andthe inlet port 158 and the cylinder 114 are in fluid communication.During the remaining 180 of rotation, the cam opens the microswitch 192and the rotary solenoid 164 repositions the switching valve 150 so thatthe porting through the valve reverses the fluid communications betweenthe cylinders and the inlet and discharge ports.

It will be noted that the portions of the cams 120 and 122 providing theproportional flow rates extend over more than 180 of the cam profilesand the cams are phased so that the profile segments providing theproportional flows overlap and simultaneously produce proportional flowsduring two portions of each shaft revolution. The switching cam 190 ismounted to the drive shaft 124 so that the reversal in the portingbetween the cylinders and the inlet and discharge ports occurs whileboth cylinders are simultaneously producing the proportional flow rates.Accordingly, by rapidly switching of the discharge port 156 between thetwo cylinders, a continuous, proportional flow to the plotting pen isproduced at all rotational positions of the drive shaft 124 and nohiatus in the flow is observed at the end of each piston stroke.

FIG. 7 shows one embodiment of the plotting pen l2 utilized in thepresent invention to deposit the pumped writing fluid on the recordingmedium 14. The plotting pen 12 is a wetting pen, that is, a pen providedwith a capillary tube at the writing tip through which the writing fluidpasses as the writing tip is drawn over a recording medium. In thisrespect, the term writing fluid" is intended to include inks, paints,photo-resists, etchants, and other fluids which are capable of providingline traces on recording mediums such as paper, photosensitive film,metal plates or other mediums on which line traces are desired.

The plotting pen 12 is composed principally of a nonmagnetic, tubularhousing 200 composed, for example, of a plastic material having a hollowcentral bore extending from a fluid feed tube 202 connecting with theflexible tube 82 (FIG. 1) to a capillary tube 204 mounted at the writingtip of the pen. The capillary tube 204 through which the writing fluidis expressed also contains a small bore, carbide insert 206 mounted inthe end of the capillary tube. The carbide tip prolongs the writinglifetime of the pen due to its high wear resistance.

Mounted within the bore of the housing 200 is a magnetic slug 210 whichis biased away from the writing tip of the pen by means of a resilientbiasing spring 212. The magnetic slug has a stainless steel filament 214extending axially from the slug through the spring 212 and into the boreof the capillary tube 204 and the carbide tip 206. As is well known inthe art, it is customary to employ a filament connected to a slug withina wetting pen to clean out dried writing fluid and open a channelthrough which fresh writing fluid can flow when the pen is first used. Apen is generally shaken manually to reciprocate the filament in thecapillary tube. Where the plotting pen is mounted securely on atransport mechanism as in the computer controlled plotting device 10,the standard procedure for shaking the pen consists of exciting the Zaxis driver 74 and solenoid 76 (FIG 2) several times in rapid successionso that the pivot arm 52 (FIG. 1) moves the pen up and down. Such anexercise produces undesirable wear on the plotting device and can damagethe writing tip of the pen as it impinges upon the recording medium 14and the underlying plotting drum 30.

In accordance with the present invention, an electrical coil 216 ismounted concentrically about the magnetic slug 210 externally of the penhousing 200. When the coil 216 is pulsed repetitively, the magnetic slug210 is oscillated in the bore of the housing 200 and the filament 214 isoscillated within the capillary tube 204 and carbide tip 206 to cleanout the dried writing fluid. In this manner, the oscillations of theslug 210 are generated without damaging the plotting drum and recordingmedium or the writing tip of the pen.

It will thus be seen that the plotting device 10 is provided withapparatus for expressing writing fluid from the plotting pen 12 at arate which is proportional to the relative movement between therecording medium and the tip of the pen. The pump 80 is designed so thatthe flow rate through the pump and the plotting pen is proportional tothe pump speed and is driven at a rate proportional to the relativespeed of the plotting pen and recording medium. The rate at which thewriting fluid is expressed from the pen is, therefore, proportional tothe plotting rate, and the quality of the line traces formed atdifferent plotting speeds remains the same. The invention has particularutility in high speed, computer-driven plotting systems where thewriting fluid normally employed is ink; however, the disclosed apparatushas an additional advantage in that fluids which are normally tooviscous to be employed in conventional plotting pens can be used in theapparatus of the present invention since the writing fluids areforciblyexpressed from the plotting pen.

It will be understood that while the present invention has beendisclosed in a preferred embodiment, numer ous modifications andsubstitutions can be had without departing from the spirit of theinvention. For example, the transporting mechanism which producesrelative movement between the pen and the recording medium may be asingle or multiple axis mechanism and it is immaterial that the pen orthe redording medium moves to produce the relative movement. It is alsonot essential that the fluid pump have the same construction as thedisclosed pump as long as the flow rate of the writing fluid can beaccurately controlled in a proportional fashion at the relatively lowflow rates of a plotting pen. Accordingly, the present invention hasbeen disclosed. in a preferred embodiment merely by way of illustrationrather than limitation.

I claim:

1. Apparatus for expressing writing fluid from a plotting pen onto arecording medium while the pen and the recording medium are movedrelative to one another in a plotting device in accordance with plottingcommands supplied to the plotting device comprising: velocity signalgenerating means connected with the plotting device for producing avelocity signal representative of the rate of movement of the pen andthe recording medium relative to one another in the plotting device; andfluid pumping means connected to the plotting pen and the velocitysignal generating means and responsive to the velocity signals forforcing writing fluid from the plotting pen at a flow rate proportionalto the rate of movement of the pen and recording medium relative to oneanother in the plotting device, the fluid pumping means including afluid piston pump having a rotatably driven drive shaft, fluiddisplacement pistons, piston drive means interconnecting the shaft andthe pistons for displacing at least one piston by an amount directlyproportional to the shaft rotation ity signal generating means forrotating the drive shaft at a rate proportional to the velocity signal.

2. Apparatus for expressing writing fluid from a plotting pen as definedin claim 1 wherein the velocity sig nal generating means includes a ratesensing circuit connected to receive the plotting commands supplied tothe plotting device.

3. Apparatus for expressing writing fluid as defined in claim 1 whereinthe piston drive means comprises drive cams interconnecting therotatably driven drive shaft and the pump pistons.

4. Apparatus for expressing writing fluid as defined in claim 3 whereinthe rotatably driven drive shaft is connected between the pump motor andthe drive cams, and the drive cams are rotatable in the pump and havecam profile segments selected to displace the pistons within thecylinders by amounts directly proportional to rotation of the driveshaft.

5. In a plotting device in which a plotting head and a recording mediumare moved relative to one another along first and second mutuallyperpendicular axes in response to plotting commands supplied to firstand second transport mechanisms associated respectively with therelative movements of the head and the recording medium along the firstand second directions, the improvement comprising: first drive motormeans connected in the first transport mechanism for receiving plottingcommands and actuating the first transport mechanism; second drive motormeans connected in the second transport mechanism for receiving plottingcommands and actuating the second transport mechanism; first ratesensing means for providing a first rate signal representative of therelative movement of the head and recording medium along the first axis;second rate sensing means for producing a second rate signalrepresentative of the relative movement of the head and the recordingmedium along the second axis; computing means connected with the firstand the second rate sensing means for producing a rate signalrepresentative of the combined rates of relative movement of the writingfluid reservoir and the plotting pen and having a drive train includinga pump drive shaft and means for displacing the individual pistons inthe pump in direct proportion to the shaft rotation or fraction thereofwhile the pistons are expelling fluid from the pump; and pump drivemeans connected to the computing means and drive shaft of the pump forcontrolling the pump to produce a flow of writing fluid from the writingtip of the recording pen at a flow rate proportional to the rate signalfrom the computing means.

6. In a plotting device, the improvement as defined in claim 5 wherein:the first rate sensing means has an input receiving plotting commandsreceived by the first drive motor means; and the second rate'sensingmeans has an input receiving plotting commands received by the seconddrive motor means.

7. In a plotting device, the improvement as defined in claim 5 whereinthe plotting pen has a capillary tube forming the writing tip of thepen, a writing fluid supply conduit connected with the fluid pump and ahousing having a bore leading from the writing piston supply conduit tothe capillary tube.

8. In a plotting device, the improvement as defined in claim 5 whereinthe writing piston reservoir and the fluid pump are mounted to theplotting head with the plotting pen.

9. In a plotting device, the improvement as defined in claim 5 whereinthe drive train of the pump has at least two rotatable drive camsmounted on a common axis and two cam-operated pumping pistons associatedrespectively with the two drive cams, and two drive cams each having camprofile segments subtending arcs about the common axis greater than andproviding positive piston displacements directly proportional to the camrotations, the cams being mounted on the common axis and rotatedtogether so that at least one of the cam profile segments displaces anassociated piston during each phase of the cam rotations.

10. In a plotting device, the improvement as defined in claim 9 whereinthe pump drive means includes a pump motor driven at a speedproportional to the rate signal from the computing means and connectedin driving relationship with the two rotatable drive cams.

1. Apparatus for expressing writing fluid from a plotting pen onto arecording medium while the pen and the recording medium are movedrelative to one another in a plotting device in accordance with plottingcommands supplied to the plotting device comprising: velocity signalgenerating means connected with the plotting device for producing avelocity signal representative of the rate of movement of the pen andthe recording medium relative to one another in the plotting device; andfluid pumping means connected to the plotting pen and the velocitysignal generating means and responsive to the velocity signals forforcing writing fluid from the plotting pen at a flow rate proportionalto the rate of movement of the pen and recording medium relative to oneanother in the plotting device, the fluid pumping means including afluid piston pump having a rotatably driven drive shaft, fluiddisplacement pistons, piston drive means interconnecting the shaft andthe pistons for displacing at least one piston by an amount directlyproportional to the shaft rotation at each rotational position of theshaft and a fluid discharge duct receiving the displaced fluid andconnected to the plotting pen; and pump motor means connected to thedrive shaft of the pump and to the velocity signal generating means forrotating the drive shaft at a rate proportional to the velocity signal.2. Apparatus for expressing writing fluid from a plotting pen as definedin claim 1 wherein the velocity signal generating means includes a ratesensing circuit connected to receive the plotting commands supplied tothe plotting device.
 3. Apparatus for expressing writing fluid asdefined in claim 1 wherein the piston drive means comprises drive camsinterconnecting the rotatably driven drive shaft and the pump pistons.4. Apparatus for expressing writing fluid as defined in claim 3 whereinthe rotatably driven drive shaft is connected between the pump motor andthe drive cams, and the drive cams are rotatable in the pump and havecam profile segments selected to displace the pistons within thecylinders by amounts directly proportional to rotation of the driveshaft.
 5. In a plotting device in which a plotting head and a recordingmedium are moved relative to one another along first and second mutuallyperpendicular axes in response to plotting commands supplied to firstand second transport mechanisms associated respectively with therelative movements of the head and the recording medium along the firstand second directions, the improvement comprising: first drive motormeans connected in the first transport mechanism for receiving plottingcommands and actuating the first transport mechanism; second drive motormeans connected in the second transport mechanism for receiving plottingcommands and actuating the second transport mechanism; first ratesensing means for providing a first rate signal representative of therelative movement of the head and recording medium along the first axis;second rate sensing means for producing a second rate signalrepresentative of the relative movement of the head and the recordingmedium along the second axis; computing means connected with the firstand the second rate sensing means for producing a rate signalrepresentative of the combined rates of relative movement of the headand recording medium along the first and second axes; a writing fluidreservoir; a plotting pen mounted to the plotting head and having awriting tip carried by the plotting head in close proximity to therecording medium; a piston pump connected between the writing fluidreservoir and the plotting pen and having a drive train including a pumpdrive shaft and means for displacing the individual pistons in the pumpin direct proportion to the shaft rotation or fraction thereof while thepistons are expelling fluid from the pump; and pump drive meansconnected to the computing means and drive shaft of the pump foRcontrolling the pump to produce a flow of writing fluid from the writingtip of the recording pen at a flow rate proportional to the rate signalfrom the computing means.
 6. In a plotting device, the improvement asdefined in claim 5 wherein: the first rate sensing means has an inputreceiving plotting commands received by the first drive motor means; andthe second rate sensing means has an input receiving plotting commandsreceived by the second drive motor means.
 7. In a plotting device, theimprovement as defined in claim 5 wherein the plotting pen has acapillary tube forming the writing tip of the pen, a writing fluidsupply conduit connected with the fluid pump and a housing having a boreleading from the writing piston supply conduit to the capillary tube. 8.In a plotting device, the improvement as defined in claim 5 wherein thewriting piston reservoir and the fluid pump are mounted to the plottinghead with the plotting pen.
 9. In a plotting device, the improvement asdefined in claim 5 wherein the drive train of the pump has at least tworotatable drive cams mounted on a common axis and two cam-operatedpumping pistons associated respectively with the two drive cams, and twodrive cams each having cam profile segments subtending arcs about thecommon axis greater than 180* and providing positive pistondisplacements directly proportional to the cam rotations, the cams beingmounted on the common axis and rotated together so that at least one ofthe cam profile segments displaces an associated piston during eachphase of the cam rotations.
 10. In a plotting device, the improvement asdefined in claim 9 wherein the pump drive means includes a pump motordriven at a speed proportional to the rate signal from the computingmeans and connected in driving relationship with the two rotatable drivecams.